The present invention relates in general to coal pulverizer controls for pulverizing coal to be used in a burner, and in particular to a new and useful control system for coal pulverizers which utilizes a calculated value for coal flowing out of the pulverizer as a function of the amount of coal known to be flowing into the pulverizer.
Coal flow to the pulverizer is controlled by varying the position of the primary air flow control damper. The coal-air mixture from the pulverizer splits into a number of burner lines (typically 8) as it leaves the pulverizer. Within the pulverizer there is a recirculating amount of coal which serves to improve drying and sizing classification of the pulverized coal particles.
The storage or recirculating load in a pulverizer varies with the grindability of the coal, the coal flow rate into the pulverizer and the primary air flow. When one or more of these variables change, the coal flow from the pulverizer will differ from the coal flow entering the pulverizer until the storage load readjusts to that required under the new conditions.
Total fuel flow to the furnace is normally controlled as a function of the summation of the feeder speeds which are calibrated from pounds of coal per feeder revolution and BTU per pound of coal which gives an indication of BTU/hour of fuel input to the furnace. Since the BTU per pound of coal may vary, a dynamic BTU correction is often applied. This dynamic BTU correction is normally developed as the integral of other steady-state throttle pressure error.
Since it is recognized that coal flow into the pulverizers does not match coal flow to the furnace except when steady-state conditions have been achieved, a variety of techniques have been utilized to second guess the pulverizer response and compensate the feeder speed back to the fuel flow control to reduce boiler upsets due to changes in fuel flow.
When starting a pulverizer, coal flow to the furnace does not immediately occur when the feeder is started. A time delay in combination with a time lag is used on introducing feeder speed into the fuel totalization circuit, therefore, according to one of these techniques.
When shutting a pulverizer down, coal flow from the pulverizer does not immediately stop when the feeder is stopped. A time lag in combination with a time delay is used to hold a value of fuel flow in the fuel totalization circuit for a period of time after the feeder is stopped unless the swing valves go closed, according to another technique.
In a further scheme, time delays and functions of equipment status are used to determine when coal flow should be expected during start-up or shutdown and to estimate when a pulverizer is clean or empty during a shutdown. Also time lags are applied on feeder speed to compensate for the pulverizer response on load changes.
In a still further scheme derivative action is used on feeder speed and primary air flow to shorten the time for pulverizer storage to reach its new value on load changes by over/under feeding the pulverizer.
Some work has also been done on developing means to measure actual coal flow from the pulverizer but no equipment appears to be available at this time. Several problems which appear to limit the application of such coal flow measurement means are:
(a) the low density of coal in the burner lines;
(b) the large number of measurements which would be required since each burner line would have to be measured; and
(c) the existing environmental conditions would tend to make sensors used for such measurements, high maintenance items.